1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, to a resistor having a serially uniform resistance and a semiconductor device using the same.
2. Description of the Related Art
When current flows through an integrated circuit (IC), heat is generated that changes the operating temperature of the IC. The temperature of an IC can also vary in response to the temperature of the external environment of the IC. A temperature change in an IC can destabilize the inherent characteristics of an element. As IC elements become smaller due to ever increasing integration of semiconductor devices, inherent operating characteristics of the element are more susceptible to change.
One of the important changes that can occur in the inherent operating characteristics of an element is a change in the resistance of an element that can be caused by a temperature change. To form a resistor, which is a passive element, polysilicon and impurity regions are formed by injecting impurities into single crystal silicon.
U.S. Pat. No. 5,187,559 provides an example of a polycrystalline silicon based resistor formed as an IC element. Table 1 is an example of the resistance temperature coefficient of such a resistor that represents the change of the resistance according to the temperature of the polysilicon and the impurity region, and the resulting sheet resistance Rs (25° C.) at an operating temperature of 25° C.
TABLE 1Resistor TypeResistance Temperature CoefficientRs(25° C.)Polysilicon−0.0352%590 Ω/squareImpurity Region+0.1014%155 Ω/square
The resistance temperature coefficient is the rate of the resistance change in response to temperature, based on the resistance at 25° C. The sheet resistance Rs (25° C.) is the resistance of a sheet-type resistor with its width equal to its length at an operating temperature of 25° C. A sheet having a length equal to its width is referred to as a square, or unit area. If a polysilicon region and an impurity region each have 1 square, this means that each sheet of polysilicon region and impurity region has a length equal to its width.
Referring to FIG. 1, polysilicon has a property such that its resistance decreases when its temperature rises, and the impurity region has a property such that its resistance increases when its temperature rises. That is, the polysilicon has a negative resistance temperature coefficient, whereas the impurity region has a positive resistance temperature coefficient. It is generally assumed that the operating temperature of an IC ranges between −45° C. and 125° C. when testing for a change in its resistance. When operating with a plurality of resistors in combination, it is difficult to maintain a uniform resistance because the resistance temperature coefficient of each resistor is different. Especially, if resistors with different resistance temperature coefficients are arranged in series, it is difficult to maintain uniform resistance throughout the series.
In order to maintain uniform resistance, a separate temperature compensation circuit can be used to adjust for the change in the resistance, which increases the number of manufacturing processes and increases overall cost. Therefore, resistors that can have different resistance temperature coefficients, but which will carry a uniform overall resistance, despite fluctuations in temperature, are needed.